Due to actualization of environmental problems in recent years, hybrid cars and electric cars have spread. Furthermore, as an air conditioner which is applicable to such a vehicle, there has been developed an air conditioner which comprises a compressor to compress and discharge a refrigerant, a radiator disposed on a vehicle interior side to let the refrigerant radiate heat, a heat absorber disposed on the vehicle interior side to let the refrigerant absorb heat, and an outdoor heat exchanger disposed on a vehicle exterior side to let the refrigerant radiate or absorb heat and which executes respective operation modes of a heating operation in which the refrigerant discharged from the compressor radiates heat in the radiator and the refrigerant by which heat has been radiated in the radiator absorbs heat in the outdoor heat exchanger, a dehumidifying and heating operation in which the refrigerant discharged from the compressor radiates heat in the radiator and the refrigerant by which heat has been radiated in the radiator absorbs heat only in the heat absorber or in this heat absorber and the outdoor heat exchanger, a cooling operation in which the refrigerant discharged from the compressor radiates heat in the outdoor heat exchanger and absorbs heat in the heat absorber, and a dehumidifying and cooling operation in which the refrigerant discharged from the compressor radiates heat in the radiator and the outdoor heat exchanger and absorbs heat in the heat absorber (e.g., see Patent Document 1).
In addition, the outdoor heat exchanger absorbs heat from outdoor air in the heating operation or the dehumidifying and heating operation, and hence frost formation occurs in the outdoor heat exchanger. When the formed frost grows in the outdoor heat exchanger, a capability to absorb the heat from the outdoor air remarkably deteriorates, and hence a defrosting operation is executed to remove the frost formation of the outdoor heat exchanger. However, during this defrosting operation, a temperature of air blown out into a vehicle interior lowers, comfort is impaired, and a power consumption increases, so that defrosting is to be minimized.
To minimize the defrosting of the outdoor heat exchanger, it is necessary to precisely judge the frost formation. Therefore, in the above publication, an outdoor air dew point temperature is calculated from an outdoor air temperature and an outdoor air humidity, it is judged whether or not a temperature of the refrigerant flowing out from the outdoor heat exchanger is lower than the outdoor air dew point temperature, and it is determined that the frost formation occurs in the outdoor heat exchanger in a case where the outdoor air dew point temperature is lower than the temperature of the refrigerant.